**3. Rheological characterization of biogas reactor fluids**

When considering the rheology for biogas reactors their viscosity is estimated to correspond to a given TS of the reactor fluid. This is mainly based on historically rheological data from sewage sludge with known TS values. However, problems may arise when using these TS relationships for other types of substrates which may impose other rheological characteristics of the reactor fluids. Furthermore, often low consideration is given to possible viscosity changes due to variation in feedstock composition etc.

Shift in the viscosity and elasticity properties of the reactor material related to substrate composition changes can alter the prerequisites for the process regarding mixing (dimension of stirrers, pumps etc. or reactor liquid circulation) and likely also foaming problems (Nordberg & Edström, 2005; Menéndez *et al*., 2006). It may also call for changes in the post treatment requirements and end use quality of the organic residue e.g. dewatering ability, pumping and spreading on arable land (Baudez & Coussot, 2001). The additions of enzymes can be used to reduce the viscosity of the substrate mixture in the digester significantly and avoid the formation of floating layers (Weiland, 2010; Morgavi *et al*., 2001). All these factors affect the total economy for a biogas plant.

In this context differences in the rheological characteristics of biogas reactor fluids as depending on substrate composition were analyzed and used as examples in this presentation.

The Ostwald model (Eq. 6), also known as the Power Law model, is applied to shear thinning fluids which do not present a yield stress (Pevere *et al*., 2006). The n-value in

The Bingham model (Eq. 7) describes the flow curve of a material with a yield stress and a constant viscosity at stresses above the yield stress (i.e. a pseudo-Newtonian fluid behaviour; Seyssiecq & Ferasse, 2003). The yield stress (0) is the shear stress () at shear rate () zero and the viscosity () is the slope of the curve at stresses above the yield stress.

When considering the rheology for biogas reactors their viscosity is estimated to correspond to a given TS of the reactor fluid. This is mainly based on historically rheological data from sewage sludge with known TS values. However, problems may arise when using these TS relationships for other types of substrates which may impose other rheological characteristics of the reactor fluids. Furthermore, often low consideration is given to possible

Shift in the viscosity and elasticity properties of the reactor material related to substrate composition changes can alter the prerequisites for the process regarding mixing (dimension of stirrers, pumps etc. or reactor liquid circulation) and likely also foaming problems (Nordberg & Edström, 2005; Menéndez *et al*., 2006). It may also call for changes in the post treatment requirements and end use quality of the organic residue e.g. dewatering ability, pumping and spreading on arable land (Baudez & Coussot, 2001). The additions of enzymes can be used to reduce the viscosity of the substrate mixture in the digester significantly and avoid the formation of floating layers (Weiland, 2010; Morgavi *et al*., 2001). All these factors

In this context differences in the rheological characteristics of biogas reactor fluids as depending on substrate composition were analyzed and used as examples in this presentation.

τ = \* (n-1) (6)

= 0 + \* (7)

0 = 0 & n = 1 Newtonian behaviour 0 > 0 & n = 1 Bingham plastic behaviour 0 = 0 & n < 1 Pseudoplastic behaviour 0 = 0 & n > 1 Dilatant behaviour

equation 6 gives fluid behaviour information according to:

**3. Rheological characterization of biogas reactor fluids** 

viscosity changes due to variation in feedstock composition etc.

affect the total economy for a biogas plant.

**2.6.2 Ostwald model** 

**2.6.3 Bingham model** 

n < 1 Pseudoplastic behaviour n = 1 Newtonian behaviour n > 1 Dilatant behaviour

0 = 0 Newtonian behaviour 0 > 1 Bingham plastic behaviour
